The field of the disclosure relates generally to digital transmission systems, and more particularly, to multi-carrier wired, wireless, and optical digital transmission systems utilizing cyclic prefixes.
Conventional digital transmission systems typically include both linear and non-linear distortion. However, for the purposes of the following discussion, use of the term “distortion” is generally intended to refer to linear distortion only. Some conventional digital transmission systems utilize orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) techniques for transmitting carrier signals using technology such as the Data Over Cable Service Interface Specification (DOCSIS), or DOCSIS version 3.1, as well as other wireless standards. OFDM implements a plurality of different subcarriers that are harmonics of a fundamental to obtain orthogonality. DOCSIS specifications typically utilize OFDM for downstream signals and OFDMA for upstream signals, and OFDM and OFDMA are complimentary.
One type of conventional digital transmission system is Radio Frequency over Glass (RFoG). RFoG is defined by the Society of Cable Telecommunications Engineers (SCTE) in SCTE 174, and transmits DOCSIS RF signals to a home, or customer premises, over fiber optics. RFoG allows cable operators to use existing Modem Termination Systems (MTS) to transmit RF over a passive optical network (PON) architecture to a modem (e.g., an optical network unit (ONU), a cable modem (CM), etc.), at the home/customer premises. The fiber optic transmission lines used in RFoG provide greater downstream and upstream bandwidth than then conventional coaxial cables. RFoG typically reduces operational expenses by allowing the substitution of passive components (e.g., splitters) for active components (e.g. amplifiers), thereby reducing the power requirements for the system, but also the reach of the system.
Both OFDM and OFDMA are known to use cyclic prefixes (CPs) in the data blocks of a transmitted digital signal. The cyclic prefix functions as a “guard time” that separates data bursts, and that allows any micro-reflection from one burst to die out before the next burst is received, thereby eliminating interference from one block to the next. CPs are commonly used in hybrid fiber coaxial (HFC) networks, where reflections are frequently known to occur, and various durations of CPs are utilized to accommodate the variety of reflections that may occur therein, thereby significantly increasing the overhead of the HFC network, because the CPs do not carry useful customer information (i.e., customer data). The CPs provide block-to-block isolation between the data block bursts of digital information, but CPs require additional resources to transmit the extra data that constitutes the CP. Such required CP data reduces the bandwidth efficiency of transmissions, thereby limiting the amount of data that can be transmitted within a given frequency band, while also requiring additional power and decreasing the battery life of system components.
DOCSIS 3.1 transmissions over an HFC network utilize a single value for an upstream CP, and the length of this single value CP is set to accommodate the longest micro-reflection that will be observed on the coaxial cable(s) of the HFC network. DOCSIS 3.1 transmissions over an RFoG network, on the other hand, require that the CP length is set long enough to activate an ONU laser of the RFoG network (sometimes referred to as an R-ONU). The RFoG ONU is typically located at the customer premises, and serves as the transport layer for RF video, voice, and DOCSIS technologies in deep fiber and fiber-to-the-home (FTTH) access networks. In many instances, the ONU also functions as or substitutes for a modem/CM. As defined in 174, where the upstream RFoG ONU laser requires 1.3 microseconds (μs) to activate and stabilize (the ONU is “always-on” downstream), upstream DOCSIS 3.1 transmissions over a RFoG network requires the upstream cyclic prefix to be greater than 1.3 μs, for the first symbol, to activate and stabilize the ONU laser. That is, within 1.3 μs, the RFoG ONU should reach and maintain steady-state stability upon turn-on.
FIG. 1 illustrates a timing diagram 100 for a conventional data burst 102 in an RFoG ONU. Timing diagram 100 depicts turn-on and turn-off durations of burst 102 such that a cyclic prefix (not shown) is generated to be long enough to accommodate the duration T1 of the leading edge of burst 102 (i.e., 1.3 μs). The T1 thus corresponds to the maximum time after application of a valid turn-on of the RF input in which an optical modulator of the ONU should achieve and maintain RF signal level stability within ±0.1 dB (e.g., observed at the output of a reference optical-to-electrical converter, not shown). The T1 duration is also considered sufficient to reach and maintain performance requirements of the noise power ratio (NPR). However, this large of and upstream CP length is wasteful of upstream transmission capacity, since the length is determined by the amount of time needed to turn-on the RFoG ONU in the first burst, but which is not needed in subsequent bursts when the ONU is already on.